1. Field of the Invention
The present invention relates to a piezoelectric drive element, and a piezoelectric drive unit (actuator) for driving a driver or a displaced body using the piezoelectric drive element.
2. Description of Related Technology
As usual driving techniques of driving lenses in an optical instrument, such as a photo-lens of a camera, a projection lens of an overhead projector, lenses of binoculars, and a lens of a copier, and of driving a typical unit having a drive section including a unit such as a plotter or an X-Y drive table, techniques disclosed in JP-A-4-069070, JP-A-11-18447, JP-A-11-44899, JP-A-11-75382, JP-A-2000-19376, JP-A-2003-141827, JP-A-2003-317410, and JP-A-2004-56951 are given. These are techniques that a piezoelectric element is slowly stretched and then quickly contracted, or quickly stretched and then slowly contracted, so that inertia force and frictional force are alternately exerted for linear driving. JP-A-4-069070 is a literature on a basic technique of the above actuator system, and JP-A-11-18447, JP-A-11-44899, JP-A-11-75382, JP-A-2000-19376, JP-A-2003-141827, JP-A-2003-317410, and JP-A-2004-56951 are literatures on how to use the system.
A mechanism of the actuator system is described with reference to FIGS. 18A to 18C. FIG. 18A is a schematic view of a drive unit, and FIGS. 18B and 18C are views showing a relationship between displacement of a piezoelectric element and time. An actuator shown in FIG. 18A is configured by a piezoelectric element 200, a shaft 202, a slider 204, and a lens 206. The piezoelectric element 200 is connected to the shaft 202 at one surface, and fixed to a body 208 at the other surface. The slider 204 is penetrated with the shaft 202, and biased to the shaft 202 by not-shown biasing elements. The slider 204 is displaced along the shaft 202 via frictional force between the slider 204 and the shaft 202 caused by the biasing elements. When the slider 204 is displaced, a lens 206 attached to an end of the slider 204 is displaced in a direction of an arrow F18a or F18b. The other end of the shaft 202 is simply held by a spring, and not fixed.
When the piezoelectric element 200 is inputted with an electric signal being asymmetrical with respect to time to be driven such that the piezoelectric element is slowly stretched and then quickly contracted as shown in FIG. 18B, when the piezoelectric element 200 is slowly stretched, the shaft 202 is moved in the direction of the arrow F18a. At that time, the slider 204 is moved with the shaft 202 by the frictional force. Then, when the piezoelectric element 200 is quickly contracted, since the slider 204 is remained at the relevant place by inertia force, and only the shaft 202 is drawn in the direction of the arrow F18b, the slider 204 is moved in the direction of the arrow F18a with respect to the shaft 202. By repeating the operation, the slider 204 is linearly driven in the direction of the arrow F18a. On the other hand, as shown in FIG. 18C, when the piezoelectric element 200 is inputted with an electric signal being asymmetrical to time to be driven such that the piezoelectric element is quickly stretched and then slowly contracted, the slider 204 is linearly driven in the direction of the arrow F18b by an effect opposite to the above effect. Since a drive mechanism of the actuator as above is simple in structure, it is practically used for an actuator for automatic focusing of a digital camera module of a cellular phone.
A lens module for a digital camera for a cellular phone is being required to inexpensively achieve improvement in performance such as high-density-pixelated optical element, zoom, automatic focus, and anti-movement of the hand. However, in the background art shown in FIGS. 18A to 18C, since the slider 204 is contacted to the shaft 202 via frictional force, they tend to be fixed to each other. In particular, since a direction of displacement of the piezoelectric element 200 is perpendicular to a direction in which fixing strength is exerted, so that displacement of the piezoelectric element 200 does not directly act on shutdown (or restraint) of the fixing strength, the piezoelectric element 200 needs to be significantly displaced to drive the slider 204, resulting in difficulties of low efficiency in driving, and difficulty in drive at low voltage.